1. Field of the Invention
The present invention relates to a method of manufacturing a plastic optical fiber, particularly to a method of manufacturing a plastic optical fiber containing a fluorescent dye.
2. Description of the Related Art
In most of ordinary methods of manufacturing optical fibers, a melt extrusion spinning method is employed in which transparent plastic is molten to be extruded from a narrow nozzle. In an example of the method of manufacturing a plastic optical fiber employing the melt extrusion spinning method, methylmethacrylate monomer is subjected to bulk polymerization, for example, for being continuously extruded as a fiber without passing through a process of being formed in pellets. Although an extruding machine used for the melt extrusion spinning method is an apparatus being large sized and expensive, the melt extrusion spinning method is suited for mass production. However, in the method, the extruding machine takes so a long time from a start of an operation to a steady operation that the method causes large losses in time and material. Therefore, an attempt of changing material such as a change in kind of resin as a material or a change in kind of additive causes losses due to the change to make the melt extrusion spinning method not always suitable for small-lot production of a wide variety of products.
In another method of manufacturing a plastic optical fiber, a rod of transparent plastic is molded before the one end thereof being heated to be drawn to a thin monofilament (hereinafter the method is referred to as the “drawing method”). Although the drawing method is not suited for mass production, it facilitates manufacturing of various kinds of optical fibers having cross sectional forms of not only being circular but also being irregularly shaped such as being rectangular. The method also facilitates change in material to be suitable for production of wide variety of products. The method of manufacturing a plastic optical fiber employing the drawing method is often applied to manufacturing a plastic optical fiber for a special purpose application such as a fluorescent fiber, or a scintillating fiber. In the drawing method, either one end portion of a transparent plastic rod is generally heated at temperatures on the order of 200 to 350° C. by a cast heater. With the heating method, the rod is heated by a part of heat from a heater conducted to the surface of the rod via air or inert gas made as a conduction medium. In addition, the surface of the rod is directly heated by far infrared rays radiated from the surface of the heater. When using a heater, a high absorption efficiency of the transparent plastic for the far infrared rays makes only the surface of the rod heated both by the heating with air taken as a heat conduction medium and by the heating with far infrared rays.
In general, plastic material has a small thermal conductivity. When a thick plastic rod is going to be heated by a heater or hot air in the drawing method, this small thermal conductivity causes that the temperature only on the surface is raised and the temperature of the central portion is not raised, thereof the heat can not conduct to the central portion. Thus, the drawing was made impossible. When the temperature of the heater or the hot air is raised for raising the temperature of the rod so far as the central portion, only the temperature of the rod surface is excessively raised to cause blowing or heat deterioration on the rod surface. This will induce defects and nonuniformity in diameter in an optical fiber manufactured from such a rod only to provide the optical fiber as being inferior in light transmission loss.
In a field of vacuum forming of plastic plates, it is known that a near infrared ray source such as a halogen lamp can be used as a heat source. The use of such a heat source, because of low absorption efficiency in plastic for near infrared rays, necessitates a supply of considerably large power to offer a problem in heating efficiency.